use typeof(); example:

> typeof "foo"
"string"
> typeof true
"boolean"
> typeof 42
"number"

So you can do:

if(typeof bar === 'string') {
   //whatever
}

use typeof();

example:

> typeof "foo"
"string"
> typeof true
"boolean"
> typeof 42
"number"

So you can do:

if(typeof bar === 'string') {
   //whatever
}

Keep in mind that, typeof is only good for returning the "primitive" types, number, boolean, object, string. You can also use instanceof to test if an object is of a specific type.

function MyObj(prop) {
  this.prop = prop;
}

var obj = new MyObj(10);

console.log(obj instanceof MyObj && obj instanceof Object); // outputs true